Optical data storage is generally limited by the optical resolution of the read/write-system. Straightforward methods of increasing the optical resolution include using a shorter wavelength and a larger numerical aperture NA, at the costs of lens complexity. Further approaches are narrowing the allowable tilt margins for the optical storage media or reducing the wavelength of the scanning laser into the blue or near-UV range. A different approach for reducing the focus spot size in an optical data storage system is using near-field optics with a high numerical aperture (NA>1). This high numerical aperture is generally achieved by help of a solid immersion lens (SIL). While conventional systems like CD, DVD or BD operate in the optical far-field regime, which is described by classical optics, the aforementioned new systems work in the optical near-field regime, which is described by near-field optics. For conventional systems the working distance, i.e. the air gap between the surface of the optical storage medium and the first optical surface of the read/write-head, usually the objective lens, is in the scale of 100 μm. In contrast, systems making use of near-field optics need a very small working distance or air gap, which is in the scale of 50 nm or less. The small air gap is necessary to ensure that evanescent waves may couple into optical storage medium. To control the distance between the read/write-head and the optical storage medium a so-called gap error signal (GES) is generated. This control method makes use of the fact that the amount of reflected light due to total internal reflection in the solid immersion lens is proportional to the size of the air gap at least in the size range used for near-field storage. An optical storage system making use of near-field optics and the gap error signal is disclosed in US 2009/0168633. Similar systems are disclosed in F. Zijp et al.: “High-Density Near-Field Optical Recording With a Solid Immersion Lens, Conventional Actuator, and a Robust Air Gap Servo”, IEEE Trans. Mag. Vol. 41 (2005), pp. 1042-1046, and C. A. Verschuren et al.: “Near-Field Recording with a Solid Immersion Lens on Polymer Cover-layer Protected Discs”, Jap. J. Appl. Phys. Vol. 45 (2006), pp. 1325-1331.